Process for the recovery of adsorbed solvents

ABSTRACT

The invention concerns a process for the recovery of a solvent adsorbed in an adsorber, or other substances which are condensable. The adsorber (3) is, at first, heated to a temperature which is below the decomposition temperature of the solvent, then the adsorber chamber (2) is sealed off from the surroundings and a high negative pressure is applied to the adsorber chamber (2), as a result of which the solvent is desorbed. During a portion of the time when this high negative pressure is applied, the temperature of the adsorber (3) is brought to a value which is above the decomposition temperature of the solvent. In spite of this, there is no decomposition under the noted conditions, however, the high temperature does enable an almost complete desorption of the solvent. Finally, the desorbed solvent is drawn off from the adsorber chamber (2) and condensed.

FIELD OF THE INVENTION

The invention concerns a process for the recovery of pollutingsubstances adsorbed in an adsorber such as, for example, solvents.

BACKGROUND OF THE INVENTION

DE PS 30 48 649 describes a process for the recovery of halogenatedhydrocarbons adsorbed in activated carbon. In this process, hot airflows through the activated carbon, whereby the adsorbed hydrocarbonsare desorbed and pass over into the air flowing through. The air flowingoff is then cool ed, which results in a condensation of the hydrocarbonsdissolved therein. By again heating and returning the air to theactivated carbon, this procedure can be repeated several times. However,strict attention must always be paid to the fact that the temperature ofthe air remain below the decomposition temperature of the halogenatedhydrocarbons. The upper limit for the temperature of the hot air flowingthrough is, for example, 120° C. with trichloroethylene and 150° C. withperchloroethylene. These values apply to normal atmosphericsurroundings. Atmospheric pressure of about 1000 mbar can thereby beconsidered to be normal atmospheric pressure.

The disadvantage of this process is that, in spite of repeating theprocedure several times, there is no complete desorption of thehydrocarbons from the activated carbon filter. In addition, the air hasa residue of hydrocarbons after the process has been completed.

Other embodiments of this process are published in the Patent Abstractsof Japan, Unexamined Applications, Section C, Volume 2, No. 59, 27.April 1978, page 458 C 78, No. 53-18504 and in EP 0 381 942. In thesepublications, the desorption of the adsorbed hydrocarbons is promoted byapplying a negative pressure to the adsorber chamber. In EP 0 381 942,the adsorber is first heated to a temperature which is below thedecomposition temperature of the adsorbed substance, the adsorberchamber is then sealed off from the surroundings and a high negativepressure is applied. Finally, the desorbed substance is drawn off fromthe adsorber chamber and the temperature and pressure ratios set in sucha way that it condenses.

This process, which is promoted by negative pressure, is in factsuperior to the first process mentioned. However, it also does notenable a complete desorption since the maximum working temperature isagain limited by the decomposition temperature of the adsorbedsubstance. Using higher temperatures, at which a substantially improveddesorption would take place, is thus not possible.

SUMMARY OF THE INVENTION

Thus, it is the object of the invention to develop the process in such away that an almost complete desorption of the adsorbed substance is madepossible.

In accordance with an embodiment of the invention, a process for therecovery of solvents adsorbed in an adsorber, is comprised of proceduralsteps of heating the adsorber to a temperature which is below thedecomposition temperature of the solvent, sealing the adsorber chamberoff from the surroundings, applying a high negative pressure to theadsorber chamber, drawing the desorbed solvent off from the adsorberchamber, and condensing the desorbed solvent by cooling at a suitablepressure characterized therein that the temperature of the adsorber isincreased to above the decomposition temperature of the solvent (atnormal atmospheric pressure) during at least a portion of the time whenthe high negative pressure is applied to the adsorber chamber.

In accordance with an embodiment, more particularly, a process for therecovery of solvents adsorbed in an adsorber is comprised of the stepsof heating the adsorber in an adsorber chamber to a temperature which isbelow the decomposition temperature of the solvent, coating the adsorberchamber off from the surroundings, applying a high negative pressurebelow the value of 10 m bar to the adsorber chamber, drawing thedesorbed solvent off from the adsorber chamber, condensing the desorbedsolvent by cooling at a suitable pressure, and increasing thetemperature of the adsorber to above the decomposition temperature ofthe solvent (at normal atmospheric pressure) for an effective periodduring at least a portion of the time when the high negative pressure isapplied to the adsorber chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention will be obtained by referring tothe detailed description below of a preferred embodiment of theinvention, with reference to the single drawing, which is a schematicillustration of an apparatus used to carry out the preferred embodimentof the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

With reference to the drawings, air mixed with an organic solvent, suchas e.g. trichloroethylene, perchloroethylene or trichloroethane, reachesthe container 2 in which the adsorber 3, e.g. an activated carbonfilter, is found, via pipe 1. After having passed through the adsorber3, the cleaned air leaves it via pipe 4. The pipes 1 and 4 each have aslide valve 5, 6. Pipe 1 has a branch to a pipe 7 which leads to avacuum pump 8. A cooling system which consists of a cooling tank 10 anda cooler 11, and which also has a separator 13, is located on the otherside of this vacuum pump. A heating device 14 is located in the adsorber3.

When the adsorber 3 is enriched with solvent, then it no longer has afiltering function and must be desorbed. To accomplish this, it isheated to a temperature which is, at first, below the decompositiontemperature of the adsorbed substance. The adsorber chamber is thenevacuated via the vacuum pump 8, which promotes desorption in a knownmanner. The typical residual pressure in the adsorber chamber is 10 to100 mbar.

EXAMPLE

According to the invention, the temperature of the adsorber 3 can now beincreased to values above the decomposition temperature of the solventby reducing the pressure further to under 1 mbar, without the solventdecomposing. This is a decidedly surprising effect. Experts in the fieldhave thusfar considered the decomposition temperature of the solvent tobe the absolute upper limit for heating.

In a successful experiment temperatures up to just under 300° C. havebeen set up, without the expected decomposition taking place. Using suchhigh temperatures produced an almost complete desorption of the solventfrom the adsorber 3 due to the thermal excitation of the solventmolecules.

Subsequent to this high-temperature phase, it is again cooled and thedesorbed substance drawn off from the adsorber chamber 2. Cooling thedesorbed substance to at least a temperature at which the substancecondenses and with a renewed increased pressure takes place in a knownmanner and is described in EP 0 281 941.

The process for the recovery of solvents adsorbed in an adsorber thus iscomprissed of the steps of heating the adsorber to a temperature whichis below the decomposition temperature of the solvent, sealing theadsorber chamber off from the surroundings, applying a high negativepressure to the adsorber chamber, drawing the desorbed solvent off fromthe adsorber chamber and condensing the desorbed solvent by cooling at asuitable pressure, the temperature of the adsorber being increased toabove the decomposition temperature of the solvent (at normalatmospheric pressure) during at least a portion of the time when thehigh negative pressure is applied to the adsorber chamber.

Preferably, the adsorber is first cooled to a temperature which is belowthe freezing point of the solvent, and during the subsequent increase intemperature of the absorber chamber, the cooling temperature is belowthe condensation point of the solvent.

The desorbed solvent may be transported into a cooling chamber 10 whereit can remain while being cooled.

The chamber in which the adsorber (3) is located may be deaerated afterit has been sealed, the deaeration taking place from the air inlet side.

If the desorbed solvent is transported into a cooling chamber forcooling, the desorbed solvent may be circulated obver a refrigerationunit.

The cooling of the desorbed solvent may take place in two steps, a firststep wherein it is cooled below the freezing point of the solvnet and,during the increase in temperature of the adsorber, it is cooled to atemperature below the condensation point of the solvent.

The negative pressure applied to the adsorber chamber may fall below thevalue of 1 mbar.

I claim:
 1. A process for the recovery of solvents of a group consistingof trichloroethylene, perchloroethylene and trichloroethane adsorbed inan adsorber comprising the steps:(a) heating the adsorber in an adsorberchamber to a temperature which is below the decomposition temperature ofthe solvent, (b) sealing the adsorber chamber off from the surroundings,(c) applying a high negative pressure below the value of 10 mbar to theadsorber chamber, (d) drawing the desorbed solvent off from the adsorberchamber, (e) condensing the desorbed solvent by cooling at a suitablepressure, and (f) increasing the temperature of the adsorber to abovethe decomposition temperature of the solvent (at normal atmosphericpressure) for an effective period during at least a portion of the timewhen said high negative pressure is applied to the adsorber chamberwhereby undercomposed solvent is recovered.
 2. A process as defined inclaim 1, in which the cooling of the desorbed solvent in step (e) takesplace in two steps, whereby it is cooled below the freezing point of thesolvent, and during the increase in temperature of the adsorber in step(f), it is cooled to a temperature below the condensation point of thesolvent.
 3. A process as defined in claim 1, including the step oftransporting the desorbed solvent into a cooling chamber, where itremains while being cooled in step (e).
 4. A process as defined in claim2, including the step of transporting the desorbed solvent into acooling chamber, where it remains while being cooled in step (e).
 5. Aprocess as defined in claim 1, including the step of de-aerating theadsorber chamber after it has been sealed, the de-aeration taking placefrom an air inlet side of the chamber.
 6. A process as defined in claim2, including the step of de-aerating the adsorber chamber after it hasbeen sealed, the de-aeration taking place from an air inlet.
 7. Aprocess as defined in claim 3, in which the desorbed solvent in thecooling chamber is circulated over a refrigeration unit.
 8. A process asdefined in claim 1, in which the cooling of the desorbed solvent in step(e) takes place in two steps, whereby it is cooled below thecondensation temperature of the solvent in the first step and below thesolidification point of the desorbed solvent in the second step.
 9. Aprocess as defined in claim 6, in which the cooling of the desorbedsolvent in step (e) takes place in two steps, whereby it is cooled belowthe condensation temperature of the solvent in the first step and belowthe solidification point of the desorbed solvent in the second step. 10.A process as defined in claim 1, in which the negative pressure appliedto the adsorber chamber falls below the value of 1 mbar.
 11. A processas defined in claim 6, in which the negative pressure applied to theadsorber chamber falls below the value of 1 mbar.
 12. A process asdefined in claim 3, including the step of de-aerating the adsorberchamber after it has been sealed, the de-aeration taking place from anair inlet.